The 360-degree video, also known as immersive video is an emerging technology, which can provide “feeling as sensation of present”. The sense of immersion is achieved by surrounding a user with wrap-around scene covering a panoramic view, in particular, 360-degree field of view. The “feeling as sensation of present” can be further improved by stereographic rendering. Accordingly, the panoramic video is being widely used in Virtual Reality (VR) applications.
Immersive video involves the capturing a scene using multiple cameras to cover a panoramic view, such as 360-degree field of view. The immersive camera usually uses a panoramic camera or a set of cameras arranged to capture 360-degree field of view. Typically, two or more cameras are used for the immersive camera. All videos must be taken simultaneously and separate fragments (also called separate perspectives) of the scene are recorded. Furthermore, the set of cameras are often arranged to capture views horizontally, while other arrangements of the cameras are possible.
The 360-degree virtual reality (VR) pictures may be captured using a 360-degree spherical panoramic camera or multiple pictures arranged to cover all filed of views around 360 degrees. The three-dimensional (3D) spherical picture is difficult to process or store using the conventional picture/video processing devices. Therefore, the 360-degree VR pictures are often converted to a two-dimensional (2D) format using a 3D-to-2D projection method, such as EquiRectangular Projection (ERP) and CubeMap Projection (CMP). Besides the ERP and CMP projection formats, there are various other VR projection formats, such as OctaHedron Projection (OHP), icosahedron projection (ISP), Segmented Sphere Projection (SSP) and Rotated Sphere Projection (RSP) that are widely used in the field.
The VR360 video sequence usually requires more storage space than the conventional 2D video sequence. Therefore, video compression is often applied to VR360 video sequence to reduce the storage space for storage or the bit rate for streaming/transmission.
The High Efficiency Video Coding (HEVC) standard is developed under the joint video project of the ITU-T Video Coding Experts Group (VCEG) and the ISO/IEC Moving Picture Experts Group (MPEG) standardization organizations, and is especially with partnership known as the Joint Collaborative Team on Video Coding (JCT-VC). VR360 video sequences can be coded using HEVC. However, the present invention may also be applicable for other coding methods.
In HEVC, one slice is partitioned into multiple coding tree units (CTU). For color pictures, a color slice may be a partitioned into multiple coding tree blocks (CTB). The CTU is further partitioned into multiple coding units (CUs) to adapt to various local characteristics. HEVC supports multiple Intra prediction modes and for Intra coded CU, the selected Intra prediction mode is signaled. In addition to the concept of coding unit, the concept of prediction unit (PU) is also introduced in HEVC. Once the splitting of CU hierarchical tree is done, each leaf CU is further split into one or more prediction units (PUs) according to prediction type and PU partition. After prediction, the residues associated with the CU are partitioned into transform blocks, named transform units (TUs) for the transform process.
Binary tree structure is more flexible than quadtree structure since more partition shapes can be supported, which is also the source of coding efficiency improvement. However, the encoding complexity will also increase in order to select the best partition shape. In order to balance the complexity and coding efficiency, a method to combine the quadtree and binary tree structure has been disclosed. According to this block partition structure, a block is firstly partitioned by a quadtree structure. Binary tree splitting can be applied to further split a block vertically or horizontally.
In conventional video coding, the block partition is applied to the picture or a region (e.g. a slice). However, a VR360 picture may contain discontinuous boundaries within the picture. The conventional block partition may not be suitable for VR360 pictures. Therefore, it is desirable to develop block partition methods that will take into account the characteristics of VR360 video sequences.